High density disk drive systems based on magnetic, magneto-optic and optical storage principles generally use a transducer system which does not, under normal operating conditions, contact the surface of the recording medium. Such non-contact transducers are known in this art as flying heads because of the principles upon which they rely to maintain a correct position with respect to the surface of the recording medium.
A brief description of how a flying head flies is now given, with reference to FIG. 15.
During operation of a disk drive, the recording medium, typically in the form of a specially coated disk of aluminum, glass or plastic, rotates at high speeds, e.g., 3600 RPM. The rotary motion of the disk 107 causes an air flow in the direction of rotation, near the surface 106 of the disk 107. The head 101 is placed by a mechanical actuator or load arm 103 in proximity with the surface 106 of the disk so that the air flow passes between the surface of the disk and the lower features of the head, thereby forming a cushion of air 108 which generates an upwards force F.sub.A on the head 101 due to air pressure in the space between the disk surface and the lower features of the head 101, with the lower features of the head defining an air bearing surface 110. The cushion of air 108 that develops between the air bearing surface 110 and the surface 106 of the disk is referred to hereinafter as an air bearing.
The flying head 101 flies at a flying height 113, defined herein as the separation distance between the air bearing surface 110 of the head 101 and the surface 106 of the disk, determined by the force balance between the air pressure F.sub.A of the air bearing 108 pushing the head 101 away from the surface 106 of the disk, and a downward force F.sub.L exerted through a spring 105 or suspension that mounts the head 101 to the load arm or actuator 103.
The force F.sub.L has a magnitude determined by the physical dimensions of the spring, the spring constant of the spring material and the deformation of the spring which occurs in operation. The upward force F.sub.A applied by the air bearing depends on the finish of the disk surface, the linear velocity of the disk surface where it passes under the head, and the shape and size of the air bearing surface of the head. Whenever F.sub.A and F.sub.L are not equal, the head experiences a net force which causes it to move in a vertical direction corresponding to the direction of the net force. When F.sub.L =F.sub.A the head experiences no net force, and hence no vertical motion.
In conventional systems, as flying height 113 increases, the air bearing 108 grows, lowering F.sub.A, while spring 105 is compressed, raising F.sub.L. The relationship between each of the forces F.sub.L and F.sub.A and flying height 113 can be determined by application of aerodynamic principles to the system configuration, which can be done by making measurements on actual systems, or physical or computer-generated models of the system. The conventional system is designed so that F.sub.L =F.sub.A at the desired flying height when the disk 107 is spinning at its normal speed. When the disk spins down, i.e., slows to a stop, insufficient air flow occurs to maintain the air bearing between head and disk. Hence, insufficient air pressure and force are generated to counteract the downward force exerted by the spring or suspension, leading to contact between head and disk. Thus, when the disk 107 slows to a stop, the head 101 may come to rest on the disk surface 106. Alternatively, the disk drive may include a mechanism that lifts the suspension 103 to prevent contact between the head and the disk when the disk spins down, but otherwise plays no role in normal disk drive operation.
Flying height 113 is one important parameter governing successful operation of a disk drive. At extremely large values for flying height 113, excessive distance from the disk can cause unacceptable functional performance, for example, an inability to discriminate high frequency signals. Close proximity of the head to the disk improves functional performance. However, at extremely small values for flying height 113, insufficient flying height or loss of separation between the head and the disk can result in aerodynamic instability, reliability problems and catastrophic product failure, e.g., a head crash which occurs when the head contacts the disk surface with sufficient force to cause damage to the head or the disk surface resulting in a loss of data. Avoiding potential damage often associated with contact between head and disk is the reason that some disk drives move their heads away from the disk surface to avoid contact when the disk spins down. The lowest height at which the head can fly without making contact with the disk surface is defined as the minimum glide height for the disk. Asperities, (i.e., microscopic bumps or roughness) in the disk surface are those features which are likely to be contacted first by the head.
One problem of disk drive manufacturing is that the physical parameters determinative of flying height. e.g., the spring characteristics (affecting load force), the design of the air bearing surface shape, manufacturing variations in the air bearing surface geometry and finish (affecting air bearing force), and the load arm position relative to the surface of the disk (affecting load force), exhibit some variation within a tolerance band which causes a corresponding variation in the load force or air bearing force and, in turn, flying height. Other sources of variation in flying height in a disk drive include variations in altitude (i.e., ambient air density), radial position of the head on the disk which varies the velocity of the air flow due to different track circumferential lengths at different track radii, and skew angle of the head relative to a line tangential to a track, all of which affect the air bearing force.
Conventionally, flying height is set by a mechanical adjustment made at the time of manufacture of a disk drive. The mechanical adjustment sets a static load force selected to provide a desired flying height under nominal conditions. For example, the static load force may be measured manually and adjusted by repositioning or bending the load arm 103. Once set, the static load force remains substantially unaltered for the life of the disk drive, despite variations in operating conditions which may cause variation in other parameters determinative of flying height. Conventional systems are also known which employ closed loop feedback control systems to maintain a substantially constant flying height. Although such systems can compensate for variations in some parameters, there remain other uncompensated tolerance errors, such as variation in the actual minimum glide height from one disk to another.
Thus, flying height in conventional disk drives cannot be set to the minimum glide height. Rather, tolerance variations such as discussed above must be considered, adding a tolerance band to the nominal or design minimum glide height of a disk when setting the actual flying height. Therefore, in order to avoid any likelihood of unwanted contact between the head and the surface of the disk, conventional systems set a nominal flying height that is greater than the largest expected actual minimum glide height.